(CNN) -- Genevieve Thiers was the oldest of seven kids, which meant she started baby-sitting almost as soon as she was born. But all the diapers, Friday nights in and lousy tips eventually paid off with millions.

Genevieve Thiers, 29, is the founder of sittercity.com.

In college, Thiers saw a pregnant woman posting fliers around campus for a baby-sitter, a cheaper method than going through an agency, which could cost in the thousands of dollars.

"I thought, 'Wow, she's desperate. Wouldn't it be great if there was one place in the country where you could go and find all of the baby-sitters and nannies in the country?' " Thiers said.